[llvm] [VectorCombine] Add constant splat handling for shuffleToIdentity (PR #92797)

[Ramkumar Ramachandra via llvm-commits]

https://github.com/artagnon commented:

Apart from renaming variables, which would be a nice-to-fix, this LGTM.

https://github.com/llvm/llvm-project/pull/92797